In ophthalmology tomographic imaging of scattering structures of the eye is of high interest. The standard technique is optical coherence tomography (OCT), employing an interferometric setup and a spatially coherent light source with a short temporal coherence length. Conventional OCT systems acquire volumetric data by scanning a focused beam over the sample and consequently, measurement speed is limited by the scanning speed. To further increase imaging speed methods with parallel detection of scattered light have been developed. Since in those tomographic ophthalmic imaging techniques that employ parallel detection no confocal imaging is used, disturbing reflexes reduce the image quality significantly.
Below are some existing ophthalmology tomographic imaging techniques:
In Fourier-Domain optical coherence tomography (FD-OCT) an interference signal over a broad spectral width is recorded in an interferometric setup. This is achieved by either detecting the signal of a broadband light source spectrally resolved (Spectral-Domain OCT) or by recording an interference signal over time, while a laser source is spectrally tuned (Swept-Source OCT). Considering the entire spectral range, the short temporal coherence length of the light source allows for an optical path length measurement of backscattered and/or reflected light from a sample in one of the interferometer arms. The path length is encoded in the interference signal, which is generated by superimposing light from sample and reference arm. Measurements of interference signals at several wavelengths allow a depth encoded profile of the sample (A-scan). The main area of application for OCT is ophthalmologic imaging, especially tomographic images of the retina (posterior eye segment) and structures in the anterior eye segment (cornea, ocular lens, iridocorneal angle).
Swept-Source OCT (SS-OCT) uses a setup with a tunable light source. While tuning the light source a wavelength-dependent interference spectrum is measured. The tuning range of the laser, which corresponds to its entire spectrum defines the coherence length of the system. The coherence length of a single wavelength is defined by the instantaneous line width. A tomographic data volume is measured conventionally by sequential data acquisition at different points, i.e., lateral scanning of the sample. Generally the sample is screened by two lateral scanners and the backscattered light is detected with a point detector. Measurement speed for sequential data acquisition might be limited by the speed of the scanners in this scenario. A phase stable detection over the whole volume is not possible in most cases. Scanning fiber based OCT systems have a confocal gating, i.e. only sample structures in the focus are illuminated and detected, while out-of-focus photons are rejected. Therefore multiple scattered photons occurring in strongly scattering media are suppressed.
A setup with a partly parallelized data acquisition is called Line-Field-OCT or Swept-Source parallel OCT (see reference 1). The backscattered light from the sample is detected in parallel in one lateral dimension, while it is scanned in the other lateral direction. The detector consists of multiple individual elements, which are arranged in a line, e.g., a line scan camera. Measurement speed is generally improved by the partial parallelization. Multiple scattered photons are not suppressed, if their last scattering event is recorded by the detector.
For a completely parallel detection in two dimensions the sample is illuminated homogeneously and spatially coherent (Full-Field Swept-Source OCT, short: FF-SS-OCT). An area scan camera is used as detector. In FF-SS-OCT the sample or rather a part of the sample is imaged onto the area camera. An advantage of this method is the increased measurement speed. While in scanning OCT systems the scanners or the tuning speed limit the measurement speed, in FF-SS-OCT the frame rate of the camera is in general the most limiting factor. All parallel detected A-scans are phase stable to each other. With this method scattered photons from all depths are detected, which are filtered in scanning systems by the confocal gating. Thus the measurement depth is larger in comparison to scanning systems, but the lateral resolution degrades out-of-focus. This degradation limits the useful measurement depth, especially at high lateral resolution. Another disadvantage of FF-SS-OCT is the detection of multiple scattered photons due to the parallel detection. Ophthalmic imaging with FF-SS-OCT has been demonstrated successfully, showing in vivo retina measurements (see reference 2)
Holoscopy is method related to FF-SS-OCT where the sample is not necessarily imaged onto the camera, but wave fields of the backscattered light from the sample are detected (see references 3 and 4). The focusing in all depth is performed in the following reconstructions. This has the advantage, that the lateral resolution does not degrade out-of-focus, but is constant over the whole volume. The reconstruction algorithm for holoscopy is also suitable for increasing the focus depth in FF-SS-OCT data.
By implementing an off-axis reference illumination in FF-SS-OCT or holoscopy it is possible to separate the signal of the interference of the sample with the reference light from its complex-conjugated signal as well as from DC and autocorrelation signals and to suppress the non-relevant signal terms. This increases the sensitivity of the imaging and avoids an overlay of multiple signal parts. (See reference 5).
All OCT techniques and related imaging methods mentioned so far are in particular suitable for scattering samples. If a sample has highly reflecting and weakly scattering parts, the strong reflections induce overexposure artifacts in the images and decrease the sensitivity of the measurements. More significantly, strong reflexes that are not within the measurement range, induce an incoherent background noise on all depth profiles/A-scans.
In other ophthalmologic imaging modalities, which have spatial incoherent illumination, strong reflexes induce image artifacts as well. This is the case for fundus cameras, where photographs of the posterior eye segment are taken, as well as for silt lamps, where all segments of the eye can be visualized enabling a variable slit shaped illumination. The artifacts caused by strong reflexes mainly lead to an overexposure of the detector, or the reflexes overshadow the actual image of the retina. The approaches to reduce those artifacts are based on the separation of illumination and detection apertures.
In conventional fundus camera setups a ring shaped aperture in the illumination light path is imaged into the plane of the pupil (References 6, 7, 8). The ring shaped illumination generates only reflexes in the outer areas, which are reflected at an angle, in a way, that they are not imaged onto the detector. The retina is illuminated divergently. The backscattered light is refracted via the optical elements of the eye and projected onto the detector. Such an aperture is not possible with coherent light, as the light will interfere on the retina and not create a constant illumination.
In conventional slit lamp setups the eye is illuminated at an angle. The angle is adjusted in a way that the backscattered light is detected, while the reflected light does not reach the detection light path (references 9, 10). When using a coherent light source the illumination of the retina is not uniform due to interference effects.
Both fundus cameras and slit lamps use conventional light sources, e.g., filament lamps or halogen bulbs. Those light sources are both spatially and temporally incoherent. Therefore there are no additional interference effects, which prevent a uniform illumination of the sample. In addition, these techniques do not provide depth information for scattering tissue.
So far there is no imaging modality that provides volumetric sample information with a parallel detection and spatially coherent light that implements reflex reduction methods to increase SNR by decreasing stray light.
As described above, existing techniques have drawbacks and disadvantages. Therefore, there is a need for an imaging system and method that allows imaging a larger portion of the eye than possible with any of today's optical imaging systems. Anterior and posterior segment can be imaged in parallel. Using parallel detection, volumetric imaging equivalent to >5 million A-scans per second (comparing to scanned OCT technology) are possible and provide very fast acquisition. The parallel detection scheme has a much higher efficiency, as it misses the confocal gating that is used in the prior art of eye inspection using optical coherence tomography. Hence a larger imaging speed can be applied within the restrictions of eye safety regulations.